1. Field of the Invention
The present invention relates to a thin film and a method of forming the same. More particularly, the present invention relates to a thin film, including multi components, composed of unit material layers which include mosaic atomic layers (MALs) composed of components constituting the thin film, and a method of forming the same.
2. Description of the Related Art
Atomic layer deposition (ALD) is a thin film deposition method which is very different from more conventional physical deposition methods such as electron beam deposition, thermal deposition, or sputter deposition. ALD is similar to chemical vapor deposition (CVD) in that chemical reactions of reaction gases are used. However, in general CVD, reaction gases are supplied at the same time and react chemically on the surface of a thin film or in the air. In ALD, different kinds of reaction gases are supplied separately by a time-sharing method, and react with the surface of a thin film. In ALD, if a different kind of reaction gas is supplied when an organic metal compound containing a metallic element (hereinafter referred to as “precursors”) is adsorbed on the surface of a substrate, the reaction gas reacts with the precursors on the surface of the substrate. As a result, a thin film is formed. Thus, precursors for ALD do not decompose by themselves at a reaction temperature, and precursors adsorbed on the surface of the substrate must be very rapidly reacted with a supplied reaction gas on the surface of the substrate. ALD can obtain the best uniformity of thickness and step coverage of the thin film from the surface reaction.
In ALD, the same kinds of precursors are adsorbed on all sites of a wafer surface on which chemisorption is possible. Even if excessive precursors are supplied, physisorption of the remaining precursors is performed on the chemisorbed precursors. Here, physisorption has less cohesion force than chemisorption. The physisorbed precursors are then removed using a purge gas. Next, different kinds of precursors are supplied and chemisorbed on the chemisorbed precursors. This process is repeated to grow a thin film on the wafer surface at a predetermined speed.
For example, in ALD using precursors A and a reaction gas B, a cycle of supplying precursors A, N2 (or Ar) purging, and supplying a reaction gas B is repeated to grow a thin film. The growth speed of the thin film represents the thickness of the thin film deposited in one cycle. As a result, the probability that molecules of precursors are adsorbed on any exposed surface is similar, regardless of the roughness of the exposed surface. Thus, if the supply of precursors is sufficient, a thin film having a uniform thickness is deposited at a constant speed regardless of the aspect ratio of the surface structure of the substrate. Also, depositing one layer at a time allows precise control of the thickness and composition of the thin film.
However, ALD also has the following problems. First, if a thin film containing three components or more is formed, the deposition rate in ALD is slower than the deposition rate in existing CVD. For example, if an SrTiO3 layer is formed by ALD, one cycle is composed of eight steps as shown in FIG. 1. Precursors containing Sr are supplied in step 10. A purge gas is supplied to purge a reaction chamber for the first time in step 20. In step 30, a reaction gas containing oxygen is supplied to oxidize the Sr atomic layer formed in step 10. A purge gas is supplied to purge the reaction chamber for the second time in step 40. Precursors containing Ti are supplied in step 50. A purge gas is supplied to purge the reaction chamber for the third time in step 60. In step 70, a reaction gas containing oxygen is supplied to oxidize the Ti atomic layer which is formed in step 50. A purge gas is supplied to purge the reaction chamber for the fourth time in step 80. Thus, the deposition rate in ALD is much slower than the deposition rate in the existing CVD, in which components contained in precursors constituting a thin film are all supplied at the same time.
Second, it is difficult to obtain satisfactory crystal phases of unit material layers constituting a thin film, and thus a subsequent thermal treatment is required. In detail, in FIG. 2, the horizontal axis represents Kelvin temperature (K) and the vertical axis represents activity. Reference numerals G1 through G11 represent activities of TiO2, BaTiO2, SrTiO3, Sr4Ti3O14, TiO2S, SrCO3, BaCO3, H2, CO2, H2O, and Sr2TiO4, respectively.
Referring to FIG. 2, each phase of SrO and TiO2 exists stably up to more than 600K if SrO and TiO2 are alternately stacked by existing ALD to deposit an SrTiO3 layer. As a result, a desired SrTiO3 layer can be formed. In other words, the SrTiO3 layer is only a combined state of SrO and TiO2. Thus, an additional thermal treatment is required to change SrO and TiO2 to a desired crystalline SrTiO3. This result is commonly applied to above a ternary thin film. Therefore, the thermal treatment is required to grow an oxide layer of a separate metallic element as a compound layer when the oxide layer is stabilized.
As described above, if a thin film containing three components or more is formed by ALD, an additional thermal treatment is required to form a thin film having a desired crystal structure. Thus, the yield of the thin film process is considerably reduced.